SVG `<path>` javascript animation not working as expected

Question

What I have achieved:

// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var length = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = length;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = length;

// Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
window.addEventListener("scroll", myFunction);

function myFunction() {
  // What % down is it?
  var scrollpercent = (document.body.scrollTop + document.documentElement.scrollTop) / (document.documentElement.scrollHeight - document.documentElement.clientHeight);
  // Length to offset the dashes
  var draw = length * scrollpercent;

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = length - draw;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.setAttribute("cx", endPoint.x);
  circle.setAttribute("cy", endPoint.y);

}

body {
  height: 2000px;
  background: #f1f1f1;
}

#circle {
  fill: red;
}

#mySVG {
  position: absolute;
  top: 15%;
  width: 100%;
  height: 1000px;
  
}

.st1 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <path  class="st1" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <circle id="circle" cx="10" cy="10" r="10"/>
  <path id="myline" class="st0" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

What I want is no matter which size or shape the SVG <path> is the growing line should be in the middle of the screen.

I tried changing the values of myline.style.strokeDashoffset = length //+newvalue - draw; and all but all it did was just ruins the consistency. so is there anyone who can help me solve this issue.?

Any help would be highly appreciatable.

Answer 1

(Update / New answer)

I think this is exactly what you want...

Update (added easing as OP wanted):

Made increase in length of #body smooth by adding transition: stroke-dashoffset ease-in-out 0.2s; to #body

Movement of #head can't be made smooth by adding transition: cx ease-in-out 0.2s, cy ease-in-out 0.2s; on #head because that will make it jump instead of moving along the path.

If you want #head to move smoothly along the track you'll have to do it by manually implementing an easing with js, which is a lot of work. So skipped that part.

let roadmapSvg = document.getElementById("roadmap-svg");
let track = document.getElementById("track");
let body = document.getElementById("body");
let head = document.getElementById("head");

let totalLength = track.getTotalLength();
let trackPoints = [];
let getTrackBounds = () => track.getBoundingClientRect();
let scaleFactor;

body.style.strokeDasharray = totalLength;
body.style.strokeDashoffset = totalLength;

function setScaleFactor(){
  scaleFactor = roadmapSvg.getBoundingClientRect().width / roadmapSvg.viewBox.baseVal.width;
}   
setScaleFactor();

function setTrackPoints(){
  let divisions = 1000;
  let unitLength = totalLength / divisions;
  trackPoints = [];
  for(let i=0; i < divisions; i++){
    let length = unitLength * i;
    let {x,y} = track.getPointAtLength(length);
    trackPoints.push({x: x*scaleFactor, y: y*scaleFactor, length});
  }
}
setTrackPoints();


function draw(){
  let currentLength = getCurrentLength();
  body.style.strokeDashoffset = totalLength - currentLength;
  headPos = track.getPointAtLength(currentLength);
  head.setAttribute("cx", headPos.x);
  head.setAttribute("cy", headPos.y);
}

function getCurrentLength(){
  let centreY = window.innerHeight / 2;
  let trackBounds = getTrackBounds();
  let currentY = centreY - trackBounds.y;
  if(currentY < 0) return 0;
  
  // if currentY is greater that track height, that means the user has scrolled pass the track (and the whole svg) in such case the animation should be completed i.e. the head should be at the final position i.e. at totalLength 
  if(currentY > trackBounds.height) return totalLength;
  
  for(let point of trackPoints){
    if(point.y >= currentY){
      return point.length;
    }
  }
  
  // (For safety) Sometimes none of the conditions match bcoz of low precision... Such situation only occurs a point very close to total length... Thus...
  return totalLength;
}

document.addEventListener("scroll", draw);

window.addEventListener("resize", () => {
  setScaleFactor();
  setTrackPoints();
  draw();
});

body {
  background: #f1f1f1;
  margin: 0;
  padding: 0 20%;
  font-family: sans-serif;
}

#roadmap-svg{
  display: block;
  max-width: 600px;
  margin: 20px auto;
  overflow: visible;
}

#roadmap-svg #head{
  fill: red;
}

#roadmap-svg #track{
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
#roadmap-svg #body{
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
  transition: stroke-dashoffset ease-in-out 0.2s;
}

.center-line{
  position: fixed;
  left: 0;
  right: 0;
  top: 50%;
  border-top: 1px solid red;
  background-color: rgba(255,255,255,0.9);
}

<div>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Tempora in eaque rem eligendi corrupti voluptate, maxime cum cumque, eius delectus minus neque, dolorem optio cupiditate ratione! Excepturi fugit culpa quo?
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<svg id="roadmap-svg" viewBox="0 0 760 300">
  <path  id="track" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" />
  <path id="body" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" />
  <circle id="head" cx="10" cy="10" r="10"/>
</svg>
<div class="center-line">Center Line</div>
<div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Sapiente officia saepe facilis? Cupiditate rem vel, quaerat ratione ipsam magnam fugiat praesentium incidunt! Eveniet cum officia impedit obcaecati id animi rerum?
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Enim tenetur possimus a neque, voluptatum reprehenderit, cum magni blanditiis quam atque dolorum veniam eveniet repellendus. Modi quibusdam maxime corrupti harum! Ullam vitae assumenda laboriosam nam officia eaque. Totam, dolorem.
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Answer 2

As per the comments and few modification to css and svg code, i was able to get the line to the center of the page, please check the working example below:

// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var length = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = length;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = length;

// Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
window.addEventListener("scroll", myFunction);

function myFunction() {
  // What % down is it?
  var scrollpercent = (document.body.scrollTop + document.documentElement.scrollTop) / (document.documentElement.scrollHeight - document.documentElement.clientHeight);
  // Length to offset the dashes
  var draw = length * scrollpercent;

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = length - draw;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.setAttribute("cx", endPoint.x);
  circle.setAttribute("cy", endPoint.y);
}

body {
  margin: 0;
  height: 1000px;
  background: #f1f1f1;
}
#circle {
  fill: red;
}
#mySVG {
  top: 15%;
  position: absolute;
  width: 100%;
}
.st1 {
  fill: none;
  stroke-dashoffset: 1;
  stroke: grey;
  stroke-width: .5;
  stroke-miterlimit: 1;
  stroke-dasharray: 2;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 1;
  stroke-miterlimit: 1;
  stroke-dasharray: 2;
}
.grid {
  position: fixed;
  width: 1px;
  height: 100%;
  background: blue;
  left: 50%;
  top: 0;
}

<div class="grid"></div>
<svg id="mySVG" viewBox="0 0 200 72" preserveAspectRatio="xMidYMin slice">
  <path class="st1" stroke-dasharray="10,9" d="m 0,5 0,4 c 0,3 2,6 5,6 l 108,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -25,0 c -3,0 -6,3 -6,6 0,3 3,6 6,6 l 35,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -18,0 c -3,0 -5,3 -5,6 l 0,4" />
</svg>

<svg id="mySVG" viewBox="0 0 200 72" preserveAspectRatio="xMidYMin slice">
  <circle id="circle" cx="0" cy="3" r="2" />
  <path id="myline" class="st0" stroke-dasharray="10,9" d="m 0,5 0,4 c 0,3 2,6 5,6 l 108,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -25,0 c -3,0 -6,3 -6,6 0,3 3,6 6,6 l 35,0 c 4,0 7,4 7,8 0,4 -3,7 -7,7 l -18,0 c -3,0 -5,3 -5,6 l 0,4" />
</svg>

Answer 3

I tried several ways to get point moving

• inside the box
• not sliding away until end
• with minimal changes to original.

I made the following changes (working example below):

• divider for percent I put 2000 (equal to 2000px, the containing body height)
• I multiplied the scroll amount from top by 18 (suitable value was a compromise whether the top or bottom becomes behaving wierdly)
• then finally I checked the percent value is not greater to one (it started to eat the 'worm' from the other end).

That's it! Maybe not the fanciest, but works.

Problematic here is the svg line is not linear from top to down, so direct element related values could not be chosen, or I at least did not find some. Thus, I ended up to simple solution and playing with parameters.

// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var length = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = length;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = length;

// Find scroll percentage on scroll (using cross-browser properties), and offset dash same amount as percentage scrolled
window.addEventListener("scroll", myFunction);

function myFunction() {
  // What % down is it?
  var scrollpercent = (document.documentElement.scrollTop * 18) / 2000;
  if (scrollpercent > 1) scrollpercent = 1;
  var draw = length * scrollpercent;

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = length - draw;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.setAttribute("cx", endPoint.x);
  circle.setAttribute("cy", endPoint.y);
}

body {
  height: 2000px;
  background: #f1f1f1;
}

#circle {
  fill: red;
}

#mySVG {
  position: absolute;
  top: 15%;
  width: 100%;
  height: 1000px;
  
}

.st1 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <path  class="st1" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <circle id="circle" cx="10" cy="10" r="10"/>
  <path id="myline" class="st0" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

Answer 4

I'd choose a different approach. I understand you want to find the point on the path that is closest to the middle of the screen. Let's do that:

1. Find the coordinates of the middle of the screen and convert them to the coordinate system of the path. The SVG API has two functions for that: .getScreenCTM() and SVGPoint.matrixTransform().
2. Find the point on the path (and its distance along the path) that is nearest to these coordinates. There is a bit of math and a search algorithm involved to do that. Mike Bostock has shown such an algorithm, and it's used here. Note that his function is open to a bit of tweaking (the precision parameter).
3. Use these data to draw the circle and the dashoffset.

It is probably a good idea to refine this by introducing a throttle for the scroll events (second variant) and then set CSS transitions to avoid visible jumps.

Transitions for the circle positioning only work with a CSS transform property. (My solution does not neccessarily move the circle along the path while transitioning. It's possible to achieve that, but goes far beyond the scope of this answer.)

var mySVG = document.getElementById("mySVG");
// Get the id of the <path> element and the length of <path>
var myline = document.getElementById("myline");
var pathLength = myline.getTotalLength();
circle = document.getElementById("circle");
// The start position of the drawing
myline.style.strokeDasharray = pathLength;

// Hide the triangle by offsetting dash. Remove this line to show the triangle before scroll draw
myline.style.strokeDashoffset = pathLength;

// throttled scroll event listener
function throttle(ms, callback) {
    var timer, lastCall=0;

    return function() {
        var now = new Date().getTime(),
            diff = now - lastCall;
        if (diff >= ms) {
            lastCall = now;
            callback();
        }
    };
}

window.addEventListener("scroll", throttle(100, myFunction));

// one initial execution
myFunction();

function myFunction() {
  var center = mySVG.createSVGPoint();
  // middle of browser viewport
  center.x = window.innerWidth / 2;
  center.y = window.innerHeight / 2;
  // transform to path coordinate system
  var matrix = myline.getScreenCTM().inverse();
  center = center.matrixTransform(matrix);

  //find nearest length on path
  var draw = getNearestLength(center);

  // Reverse the drawing (when scrolling upwards)
  myline.style.strokeDashoffset = -draw - pathLength;

  //get point at length
  endPoint = myline.getPointAtLength(draw);
  circle.style.transform = "translate(" + endPoint.x + "px, " + endPoint.y + "px)";
}

function getNearestLength(point) {
  var precision = 8,
      best,
      bestLength,
      bestDistance = Infinity;
  // linear scan for coarse approximation
  for (var scan, scanLength = 0, scanDistance; scanLength <= pathLength; scanLength += precision) {
    if ((scanDistance = distance2(scan = myline.getPointAtLength(scanLength))) < bestDistance) {
      best = scan, bestLength = scanLength, bestDistance = scanDistance;
    }
  }
  // binary search for precise estimate
  precision /= 2;
  while (precision > 0.5) {
    var before,
        after,
        beforeLength,
        afterLength,
        beforeDistance,
        afterDistance;
    if ((beforeLength = bestLength - precision) >= 0 && (beforeDistance = distance2(before = myline.getPointAtLength(beforeLength))) < bestDistance) {
      best = before, bestLength = beforeLength, bestDistance = beforeDistance;
    } else if ((afterLength = bestLength + precision) <= pathLength && (afterDistance = distance2(after = myline.getPointAtLength(afterLength))) < bestDistance) {
      best = after, bestLength = afterLength, bestDistance = afterDistance;
    } else {
      precision /= 2;
    }
  }
  return bestLength;

  function distance2(p) {
    var dx = p.x - point.x,
        dy = p.y - point.y;
    return dx * dx + dy * dy;
  }
}

body {
  height: 2000px;
  background: #f1f1f1;
}

#circle {
  fill: red;
}

#mySVG {
  position: absolute;
  top: 15%;
  width: 100%;
  height: 1000px;
  
}

.st1 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: grey;
  stroke-width: 4;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
}
.st0 {
  fill: none;
  stroke-dashoffset: 3px;
  stroke: red;
  stroke-width: 5;
  stroke-miterlimit: 10;
  stroke-dasharray: 20;
  transition: stroke-dashoffset 0.2s;
}

#circle {
   transform: translate(10px, 10px);
   transition: transform 0.2s;
   }

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <path  class="st1" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>

<svg id="mySVG" viewBox="0 0 60 55" preserveAspectRatio="xMidYMin slice" style="width: 6%; padding-bottom: 42%; height: 1px; overflow: visible">
  <circle id="circle" cx="0" cy="0" r="10"/>
  <path id="myline" class="st0" stroke-dasharray="10,9" d="M 20 0 v 20 a 30 30 0 0 0 30 30 h 600 a 40 40 0 0 1 0 80 h -140 a 30 30 0 0 0 0 60 h 200 a 40 40 0 0 1 0 80 h -100 a 30 30 0 0 0 -30 30 v 20" /> Sorry, your browser does not support inline SVG.
</svg>